The present disclosure relates in general to optical system monitoring, and, more particularly, to a method and system for sensing optical fiber temperature.
Optical fibers increasingly constitute the chief means for transmitting information through the world""s telecommunications network. Certain characteristics of an optical fiber can also be used to generate information rather than just transmit it. Specifically, the temperature of an optical fiber affects the amount and wavelength of light that will be scattered in response to a transmitted pulse. Careful measurements of scattered light can therefore be used to determine the temperature at points along an optical fiber.
A time-limited pulse of light with an electromagnetic spectrum of average wavelength xcex can be sent through an optical fiber. As the pulse traverses the fiber, backward scattered light is produced. Two types of backward scattered light are of particular interest: Stokes light and anti-Stokes light. Stokes light constitutes an electromagnetic spectrum having an average wavelength above xcex. Anti-Stokes light constitutes an electromagnetic spectrum having an average wavelength below xcex. Other types of backward scattered light are also produced at wavelengths outside the Stokes and anti-Stokes spectra. The width of the Stokes and anti-Stokes spectra, as measured by the difference in wavelength between the points of 50% intensity, is often much greater than the spectrum width of the time-limited pulse, especially if that pulse is produced by a laser.
The Stokes and anti-Stokes light travels to the end of the fiber at which the pulse was introduced. The location from which the backward scattered light originated can be determined by the time between the introduction of the pulse and the receipt of the light. After a pulse is introduced into the fiber, backward scattered light is continuously received and time functions of the total intensity across the Stokes and anti-Stokes spectra can be determined. The temperature of a point in the fiber has a known relationship to the ratio of the anti-Stokes light produced at that point to the Stokes light produced at that point. Increasing the measurement accuracy of Stokes and anti-Stokes intensity as a function of time, increases the accuracy of the resulting calculation of temperature as a function of position in the fiber.
A temperature sensing fiber of greater sensitivity can be used to monitor conditions anywhere a fiber can be located. For example, a fiber can be run along a power cable to detect degradation at specific points along the cable. Fiber could also be used to detect fires along the length of the fiber, even over long distances.
U.S. Pat. No. 5,113,277 discloses a Fiber Optic Distributed Temperature Sensor System. The ""277 patent contemplates introducing a light pulse from a light source into a fiber. The scattered light is then divided by wavelength with detectors positioned to receive the Stokes light and anti-Stokes light, respectively. The measurements made by the detectors are then introduced into an equation to determine the temperature at a certain distance.
The ""277 patent discloses several different configurations for alternatively reflecting or transmitting various wavelengths of light in order to guide the pulse into the fiber and guide the Stokes and anti-Stokes responses to different detectors. Each of those configurations involves transmitting the pulse from the light source through one or more lenses within the optical wavelength division demultiplexer. The imperfections of conventionally available transmitting lenses cause an appreciable percentage of light to scatter rather than be transmitted. The scattered light is comparable in intensity to the Stokes and anti-Stokes reflections because the light source pulse is of much greater intensity than the back scattered radiation. The scattered light is spread over a wide angle. The scattered light introduces error into measurements and reduces the specificity of resulting calculations.
An additional difficulty with the conventional approach of using transmitting lenses is caused by the back scattering of Rayleigh light. Unlike Stokes and anti-Stokes light, Rayleigh light has wavelengths in the electromagnetic spectrum of the pulse from the light source. As with the source pulse, an appreciable percentage of the Rayleigh light is scattered rather than transmitted by the transmitting lenses. Measurement difficulties arise because the intensity of Rayleigh back scattered light is greater than the intensity of Stokes and anti-Stokes scattered light.
A method and system sensing optical fiber temperature is disclosed. None of the advantages disclosed, by itself, is critical or necessary to the disclosure.
A system for measuring optical fiber temperature includes a laser that produces at least a first electromagnetic spectrum at an output. A first optical filter includes a side that receives the first electromagnetic spectrum from the laser and reflects it in a specific direction. The first optical filter transmits a second electromagnetic spectrum having an average wavelength below that of the first and also transmits a third electromagnetic spectrum having an average wavelength above that of the first. A second optical filter is positioned to receive light received from the specific direction and transmitted through the first optical filter. The second filter reflects the second electromagnetic spectrum and transmits the third electromagnetic spectrum. One detector is positioned to received light reflected from the second optical filter. A second detector is positioned to receive light transmitted through the second optical filter.
A more specific system is also provided in which a third optical filter is positioned to receive light reflected from the second optical filter, transmits light in the second electromagnetic spectrum, but does not transmit light in the first or third electromagnetic spectra. A another more specific system is provided in which the optical filters are mounted in a case in fixed relation to each other and to collimating lenses through which the laser and the detectors are coupled to the filters. The case defines a sealed cavity. The air in the cavity is preferably as dust free as possible. As an alternative the cavity could be maintained at below atmospheric pressure or filled with a fluid other than air.
A method is provided for measuring optical fiber temperature. The method includes producing a first electromagnetic spectrum at the output of a laser. The first spectrum is then reflected off a first optical filter and guided into an optical fiber. Electromagnetic radiation at wavelengths falling into second and third spectra are then received from the fiber at the first optical filter. The second spectra has an average wavelength below the average of the first. The third has an average above the first. The first optical filter transmits the second and third spectra that are then received at a second optical filter. The second optical filter reflects the second spectrum, but transmits the third spectrum. Two detectors are positioned so that each receives one of the second and third spectra. A more specific embodiment of the method reflects at least 99.8% of the first spectra at the first optical filter, transmits at least 80% of the second and third spectra at the first optical filter, reflects at least 95% of the second spectra at the second optical filter, and transmits at least 80% of the third spectra at the second optical filter.
It is a technical advantage of the disclosed methods and systems that light spectra having average wavelengths both above and below the average wavelength of the source light are measured.
It is also a technical advantage of the disclosed methods and systems that the source light is reflected rather than transmitted to reduce scattered light.
Another technical advantage of the system and method disclosed is that nonreflected light scatters over a much smaller solid angle as compared to nontransmitted light. As a result, the nonreflected light can be contained more easily and even used to trigger a timing measurement.
Another technical advantage of the system and method disclosed is that a ratio of the intensity of light in the spectra with lesser average wavelength can be compared with the intensity of light in the spectra with greater average wavelength to determine temperature.
Another technical advantage of the system and method disclosed is that the time difference between the source light pulse and measurements can be used to determine the distance at which the temperature is calculated.
Another technical advantage of the system and method disclosed is that Rayleigh scattered light is reflected rather than transmitted to reduce scattered light.
Another technical advantage of the system and method disclosed is that collimating lenses reduce intensity loss.
Other technical advantages of the present disclosure will be readily apparent to one skilled in the art from the following figures, descriptions, and claims. Various embodiments of the invention obtain only a subset of the advantages set forth. No one advantage is critical to the invention. For example, one embodiment of the present invention may only provide the advantage of measuring light spectra having average wavelengths both above and below the average wavelength of the source light, while other embodiments may provide several of the advantages.